The present invention relates generally to high performance wireless broadband communications systems, and more specifically to an improved technique for performing channel estimation in wireless broadband communications systems.
U.S. patent application Ser. No. 11/115,943 filed Apr. 27, 2005 entitled MULTIPLE INPUT MULTIPLE OUTPUT (MIMO) WIRELESS COMMUNICATIONS SYSTEM (the “'943 application”) and assigned to the same assignee of the present invention discloses a high performance wireless broadband communications system that is configurable as a line-of-sight (LOS) or non-line-of-sight (NLOS) system. As disclosed in the '943 application, the wireless broadband communications system includes at least one transmitter disposed at one end of a communications link, and at least one receiver disposed at the other end of the link. The transmitter is operative to send data signals over one or more communications channels using specified space-time coding and modulation techniques. The receiver is operative to capture the transmitted signals, and to employ specified signal processing techniques to decode and demodulate the signals to recover user data. Both the LOS and NLOS configurations of the disclosed system employ adaptive modulation techniques to adjust selected transmission parameters such as the coding rate and the modulation mode, thereby compensating for changes in channel characteristics that may adversely affect the quality and/or the rate of data transmission.
The wireless broadband communications system disclosed in the '943 application typically compensates for changes in channel characteristics based at least in part upon the states of signal propagation paths corresponding to the respective channels. For example, to determine the states of the propagation paths, the disclosed system typically employs pilot carriers in an orthogonal frequency division modulation (OFDM) waveform to transmit pilot signals along the respective paths over orthogonal channels. To assure that channel state information can be determined separately for each orthogonal channel, separate sets of frequencies or “tones” may be employed when transmitting the pilot signals over the respective channels. The transmitter disposed at one end of the communications link operates to modulate and transmit the pilot signals over the respective channels, and the receiver disposed at the other end of the link operates to receive and demodulate the pilot signals, which provide a phase reference for data carriers in the OFDM waveform. The disclosed system can employ the pilot signals to obtain an estimate of the impulse response of each channel, which may subsequently be used by the system to select the transmission parameters for a given communications session. In addition, the system can employ the phase reference provided by the pilot signals to increase the accuracy of signal demodulation at the receiver.
However, the use of pilot carriers in an OFDM waveform to perform channel estimation or to provide a phase reference for signal demodulation in wireless broadband communications systems can be problematic because, due to various factors, one or more of the pilot tones may be suppressed, corrupted, or otherwise missing at the receiver. One such factor is adjacent channel interference (ACI), which can prohibit the transmission of significant power in one or more of the pilot tones near the edges of a frequency band. Other factors include processing noise and analog errors, which can corrupt pilot tones at zero frequency (DC). Corrupted, suppressed, or missing pilot tones can cause truncation effects, which may result in distortions of both the channel impulse response and the channel frequency response. For example, such truncation effects may be manifested as a “blurring” of the channel impulse response. If this blurring of the channel impulse response extends beyond a specified window of the impulse response, then the corresponding channel frequency response may exhibit “ringing” near the edges of the frequency band. In addition, if the number of pilot tones is reduced to enhance spectral efficiency, then the distortions of the channel impulse response and the channel frequency response may be increased. Because such corrupted, suppressed, or missing pilot tones can cause distortions of the channel estimate, it can be difficult at best to select optimal parameters for signal transmission and to assure high accuracy of signal demodulation in wireless broadband communications systems.
It would therefore be desirable to have an improved technique for performing channel estimation in wireless broadband communications systems.